Played around with my stones and came up with two designs of the same theme:
Aqua(rock)scape 1: Smaller stones means more room, but perhaps a bit too much on top? I could always raise them by stacking some rocks underneath.
Aquascape 2: The larger stone gives a much more profound, sharper image. But it also is very large, caved in in the back, and hard to position stably.
I'm adding a poll now, but if you have further suggestions please leave a comment.